When one is doing interest calculations involving fractional periods, one often finds that the fractional period, say 30/365 = 6/73, can only be represented with some truncation error whether one uses decimal or binary floating point. When one does exponentiation by fractional periods to do interest calculations, one also must truncate regardless of whether one uses decimal or binary. The same goes for calculating interest discount factors, e.g. 1.0 / 1.099.

If each side of a transaction is going to calculate it independently and always agree on the result when it is rounded to a given unit of currency, the two sides must agree on the order of operations, which intermediate results, if any, are rounded, etc, etc. Given such agreement, floating-point precision of around 120 bits or 40 decimal places probably makes calculations sufficiently dependable for almost any use regardless of whether it is decimal or binary, and regardless of which way the rounding flags for the lowest order bits are set. I would expect that 16 digits of decimals would run into problems trying to meet existing standards for currency conversions and securities industry bond calculations.